An audio amplifier circuit is used to amplify a weak audio signal to drive an electro-acoustic transducer such as a speaker or a headphone. FIG. 1 is a circuit diagram of an audio output device 100r. The audio output device 100r has, in addition to an electro-acoustic transducer 102, an audio amplifier integrated circuit (IC) 200r, filters 104P and 104N and snubber circuits 106P and 106N, which are configured to be symmetrical to a positive electrode and a negative electrode of the electro-acoustic transducer 102, and the electro-acoustic transducer 102 is bridged transless/bridge-tied load (BTL)-connected to the audio amplifier IC 200r. 
The audio amplifier IC 200r has an OUTP terminal and an OUTN terminal. The filter 104P is installed between the positive electrode terminal (+) of the electro-acoustic transducer 102 and the OUTP terminal, and the filter 104N is installed between the negative electrode terminal (−) of the electro-acoustic transducer 102 and the OUTN terminal. The filter 104P and 104N are primary filters each of which has a series inductor L1 and a shunt capacitor C1.
The audio amplifier IC 200r has class D amplifiers 202P and 202N, drivers 204P and 204N, and a pulse modulator 206. The pulse modulator 206 receives an analog or digital audio signal S1 and pulse-modulates the same to generate pulse signals S2P and S2N.
The driver 204P drives the class D amplifier 202P according to the pulse signal S2P. Similarly, the driver 204N drives the class D amplifier 202N according to the pulse signal S2N.
FIG. 2 is a waveform view of the audio output device 100r of FIG. 1, which operates in a differential manner. In the present specification, the waveform view and the vertical axis and the horizontal axis of the time charts are appropriately enlarged and reduced to facilitate understanding of the present disclosure and also simplified to facilitate understanding of each waveform view shown.
Here, in order to facilitate understanding, a case where a triangular wave and the audio signal S1 are compared to generate pulse signals S2P and S2N will be described. In a class D amplifier based on a differential scheme, the pulse signals S2P and S2N are reverse-phased. As a result, a voltage Vo+ of the OUTP terminal and a voltage Vo− of the OUTN terminal become differential signals, so that a maximum amplitude thereof is double a source voltage VDD of the class D amplifiers 202P and 202N.
In the class D amplifier based on the differential scheme, the filters 104P and 104N serve as low-pass filters (LPFs) for removing a switching frequency of a differential signal Vo to reproduce the original audio signal S1.
Recently, a filterless scheme has been employed in the place of the class D amplifier based on the differential scheme described with reference to FIG. 2. FIG. 3 is a waveform view of the audio output device 100r operating in a filterless manner. In a filterless operation, the audio signal S1 and a triangular wave are compared to generate a pulse signal S2P, and an inverted signal #S1 of the audio signal S1 and a triangular wave are compared to generate a pulse signal S2N. In this filterless scheme, an amplitude of the differential signal Vo applied to the electro-acoustic transducer 102 is ½ of that of the differential scheme of FIG. 1, but the LPFs for removing a switching frequency are not required. However, in order to suppress an unnecessary electromagnetic interference (EMI), the filters cannot be removed, and in the filterless scheme, the filters 104P and 104N serve as EMI removal filters.
When the audio output device 100r of FIG. 1 is operated in a filterless manner, in a state where there is a big difference between duty ratios of the OUTP and OUTN, that is, in a state where a current of the electro-acoustic transducer 102 is large, the output voltages Vo+ and Vo− overshoot. In order to suppress overshoot, the snubber circuits 106P and 106N are additionally required for each of the OUTP and OUTN terminals, which causes an increase in the number of components of the circuit.